1. Field of the Invention
The invention relates generally to radio frequency receivers, and more specifically to multiple band global positioning system (GPS) receivers used for navigation.
2. Description of the Related Art
GPS satellites transmit data at two radio frequency (RF) system carrier frequencies: 1575.42 MHz (L1) and 1227.6 MHz (L2). GPS data from both carriers can be used to increase the position accuracy, and to provide carrier selectivity in case of interference or jamming of one of the carriers.
A GPS receiver designed to receive the L1 and/or L2 carriers requires a method for receiving both signals simultaneously or efficiently switching between the signals. One solution is to duplicate all receiver parts and functions for the L1 and L2 bands. However, for low-power portable receivers, it is desirable to integrate the L1 and L2 functions as much as possible, to minimize the number of RF functions and power dissipation.
It has been known for LI/L2 receivers to use parallel RF paths and/or RF switching of the input and/or local oscillator (LO) signals. For example, U.S. Pat. No. 5,883,597 discloses an LI/L2 GPS receiver in which the LO is switched between three frequencies to select “L1 only,” “L2 only” or “L1 and L2.” However, this requires the LO to be tunable over a very wide frequency range of about 696 MHz, from approximately 1054 MHz to 1750 MHz, which makes on-chip integration difficult. Further, due to practical design limitations, this may require switching between two or three tuned oscillators, which may result in excessive power consumption for multiple voltage controlled oscillators (VCOs). Also, in the “L1 and L2” mode, this receiver may suffer a 3 dB noise penalty due to image noise. Switching of the LO signal may also require resynchronization of tracking loops, which reduces receiver response time for time sensitive applications.
U.S. Pat. No. 5,678,169, for example, discloses an L1/L2 receiver in which the VCO and LO frequency is fixed exactly halfway between the LI and L2 carriers, as in the “L1 and L2” mode of the above referred-to receiver. This receiver uses switched L1 and L2 filters which eliminate the problem of the 3 dB image noise. However, this receiver may not be capable of true simultaneous LI and L2 detection, since the L1/L2 selection is done by RF switches before the mixer.
U.S. Pat. Nos. 5,040,240 and 5,736,961, for example, disclose L1/L2 receivers which use parallel RF paths for the downconversion. U.S. Pat. No. 5,040,240 uses a common VCO with a series of different dividers and multipliers for the L1 and L2 downconversions. However, due to the duplication of RF functions, these methods are not optimum for high integration and low-power.
Therefore, those concerned with the development and use of improved dual frequency carrier signal receiver systems and methods have recognized the need for improved systems and methods for enabling simultaneous dual frequency capabilities without requiring radio frequency switches or local oscillator switching.
Accordingly, the present invention fulfills these needs by providing efficient and effective systems and methods for simultaneously receiving or switching between dual frequency carrier signals in a highly integrated, low power receiver.